Biology Concept 28.2: Protists and Mitochondria

Eukaryotic Tree and Protists

• The root of the eukaryotic tree is unknown, and new groups of protists have been discovered, but their relationships to supergroups are unresolved.
• Examples of unresolved groups include haptophytes, cryptophytes, and hemimastigophores.

Excavates

• Excavates are characterized by their cytoskeleton and some have an "excavated" feeding groove on one side of the body.
• The excavates include three monophyletic groups: diplomonads, parabasalids, and euglenozoans.
• Diplomonads and parabasalids lack plastids and have reduced mitochondria, and most live in anaerobic environments.
• Diplomonads have two equal-sized nuclei and multiple flagella, and many are parasites, such as Giardia intestinalis.
• Parabasalids have reduced mitochondria, called hydrogenosomes, that generate some energy anaerobically, and release hydrogen gas as a by-product.

SAR Supergroup

• SAR is a monophyletic supergroup named for the first letters of its three major clades: stramenopiles, alveolates, and rhizarians.
• Stramenopiles include some of the most important photosynthetic organisms on Earth, and most have a "hairy" flagellum paired with a "smooth" flagellum.

Stramenopiles

• Diatoms are unicellular algae with a unique two-part, glass-like wall of silicon dioxide, and are an important group of stramenopiles.
• Diatoms are so abundant and widespread that their photosynthetic activity affects global CO2 levels, and promoting diatom blooms by fertilizing the ocean with essential nutrients is a proposed approach to reduce atmospheric CO2 levels.
• Oomycetes are related to plastid-bearing groups, but do not have plastids or perform photosynthesis, and acquire nutrients through parasitism or decomposition.
• Brown algae are another important group of stramenopiles.

Alveolates

• Alveolates have membrane-enclosed sacs (alveoli) just under the plasma membrane, and three clades included in the alveolates are the dinoflagellates, the apicomplexans, and the ciliates.
• Dinoflagellates are abundant components of marine and freshwater phytoplankton, and have two flagella housed in the grooves of armor-like cellulose plates that surround the cell.
• Dinoflagellate blooms cause "red tides" where the water appears brownish red or pink due to the carotenoids present in their plastids, and are toxic and can cause massive kills of invertebrates and fishes.

Green Algae

• Green algae are named for their green chloroplasts, which are structurally and chemically similar to those found in plants, and form a paraphyletic group that includes the charophytes and the chlorophytes.
• Charophytes include the algae most closely related to plants, and most chlorophytes live in fresh water, although there are many marine and some terrestrial species.
• Larger size and greater complexity evolved in green algae by three different mechanisms: formation of colonies, formation of true multicellular bodies, and repeated division of nuclei.

Unikonts

• Unikonts are a supergroup that includes protists that are closely related to fungi and animals.

Protists in Ecological Communities

• Protists are found in diverse aquatic and moist terrestrial environments, and play two key roles in their habitats: that of symbiont and that of producer.
• Some protist symbionts benefit their hosts, while others are parasites.
• Photosynthetic protists are producers, and are limited by nutrients, and populations explode when nutrients are added, leading to major ecological consequences.
• Phytoplankton communities rely on upwelling of cold, nutrient-rich water from the below, and warm surface water acts as a barrier to upwelling.